1. Field of the Invention
This invention generally relates to scanners operative for electro-optically reading indicia having parts of different light reflectivity, for example, bar code symbols, and, more particularly, to control circuitry in such scanners to enable the scanner to adapt to specific application environments and symbol readability conditions. The invention also relates to an optical design for eliminating collection optical components in the return path along which light reflected off the indicia travels.
2. Description of the Related Art
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light-reflecting characteristics. The readers and scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. Scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297; 4,409,470; 4,760,248; 4,896,026, all of which have been assigned to the same assignee as the instant application.
As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in a hand-held, portable laser scanning head supported by a user, which is configured to allow the user to aim the head, and more particularly, a light beam, at a target and a symbol to be read.
The light source in a laser scanner bar code reader is typically a gas laser or semiconductor laser. The use of semiconductor devices as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a focusing optical assembly, to form a beam spot of a certain size at the target distance. It is preferred that the cross section of the beam spot at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
The bar code symbols are formed from bars or elements typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique "start" and "stop" character is used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
For the purpose of our discussion, characters recognized and defined by a symbology shall be referred to as legitimate characters, while characters not recognized and defined by that symbology are referred to as illegitimate characters. Thus, an arrangement of elements not decodable by a given symbology corresponds to an illegitimate character(s) for that symbology.
In order to increase the amount of data that can be represented or stored on a given amount of surface area, several new bar code symbologies have recently been developed. One of these new code standards, Code 49, introduces a "two-dimensional" concept by stacking rows of characters vertically instead of extending the bars horizontally. That is, there are several rows of bar and space patterns, instead of only one row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239, which is hereby incorporated by reference. Another structure, known as "PDF 417", is described in U.S. patent application Ser. No. 461,881.
In the scanning systems known in the art, the light beam is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanner operates by repetitively scanning the light beam in a line or series of lines across the symbol by means of motion of a scanning component, such as a mirror, disposed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line across and past the symbol, or scan the field of view of the scanner, or do both.
Bar code reading systems also include a sensor or photodetector which functions to detect light reflected or scattered from the symbol. The photodetector or sensor is positioned in the scanner in an optical path so that it has a field of view which extends across and slightly past the symbol. A portion of the light which is reflected or scattered off the symbol is detected and converted into an electrical signal. Electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal operated by the photodetector may be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded based upon the specific symbology used by the symbol into a binary representation of the data encoded in the symbol, and subsequently to the alphanumeric characters so represented.
The decoding process in known bar code reading systems usually works in the following way. The decoder receives the pulse width modulated digital signal from the bar code reader, and an algorithm implemented in software attempts to decode the scan. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator or a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a completely decoded scan is achieved or no more scans are available.
It must be recognized that the overall performance of a scanning system for reading symbols is a function not only of the optical, but also of the electronic sub-system. A measure of the overall performance of a bar code reader is the ability to resolve narrowest elements and to decode symbols located perhaps hundreds of inches away from the reader. The optical subsystem will focus the beam to have a certain measurable spot size, but the electronic sub-system, and particularly the analog signal processing circuitry, also has a role to play in contributing to the detection and spot size. One method of measuring the contribution of the circuitry is by the concept of effective spot size which was introduced by Eric Barkan and Jerome Swartz in the following two articles:
1. "Advances in Laser Scanning Technology", Proceedings of The International Society For Optical Engineering, Volume 299, Aug. 27-28, 1981.
2. "System Design Considerations in Bar-Code Laser Scanning", Optical Engineering, Volume 23, No. 4, Pages 413-420, July/August, 1981
The concept of effective spot size was defined in such articles by the following equation: EQU Weff (2)=.sqroot.Wept.sup.2 (2)+Wep.sup.2 (2)
wherein:
W.sub.opt is the spot size of the focused beam at the focal plane due solely to the optical system; and
W.sub.ei is the addition to the spot size caused by the electrical system
The W.sub.ei parameter is a function of the frequency bandwidth or the time constant of the analog system processing circuitry, as well as a function of the laser beam spot velocity at the focal or scanning plane. With increasing distance from the housing, the contribution of W.sub.ei, results in an increase in the value of W.sub.eff, thereby degrading overall system performance at such far-out distances. At too long a far-out distance, the symbol can no longer be read.
Prior to the present invention, the adjustment of scanning parameters was made on a piecemeal basis, by independently adjusting a single parameter like beam intensity or amplifier gain. No consideration was given to the simultaneously adjusting several different optical and electrical parameters together so that the readability of a symbol at a given distance or range of distances or a particular application or operational object (e.g., maximum working range) is optimized.